The use of Radio Frequency Identification (RFID) tags are quickly gaining popularity for use in the monitoring and tracking of items. RFID technology allows a user to remotely store and retrieve data in connection with an item utilizing a small, unobtrusive tag. As an RFID tag operates in the radio frequency (RF) portion of the electromagnetic spectrum, an electromagnetic or electrostatic coupling can occur between an RFID tag affixed to an item and an RFID reader, which is capable of reading the tag. This coupling is advantageous, as it precludes the need for a direct contact or line of sight connection between the tag and the reader.
In some currently used passive and semi-passive RFID tags, during the ‘read’ cycle, the reader generally transmits a continuous unmodulated carrier signal. A distant RFID tag includes a RF switch connected to the tag's antenna, which repetitively alternates its state at a rate called the ‘backscatter link frequency.’ This RF switch effectively modulates the carrier signal received from the transmitter, creating sidebands within the tag surrounding the carrier frequency, and separated from the carrier frequency by the backscatter link frequency. These sidebands are re-radiated by the tag's antenna, and are recovered by the reader. The above description is one typical way in which the tag communicates information to the reader. The tag does not create RF power, but instead modulates incoming RF power from the reader's transmitter, and in so doing, converts some of that incoming power to sideband frequencies which can be separately recovered by the reader. These backscatter sidebands only exist when (and because) the reader is transmitting.
In some readers, the antenna configuration is ‘monostatic’ which means that the sidebands created by the distant tag are recovered via the same antenna that the reader's transmitter uses to transmit the carrier signal. In a monostatic reader, a power amplifier is connected to a signal splitter, which is connected to an antenna and a low noise amplifier. In some other readers, the antenna configuration is ‘bistatic’ which means that the sidebands created by the distant tag are recovered by the reader via a separate antenna. A bistatic reader differs from a monostatic reader in that each reader has a power amplifier, low noise amplifier, and antenna, but in the bistatic reader, the power amplifier is connected directly to a transmit antenna, a separate receive antenna connects directly to the low noise amplifier, and there is no signal splitter.
In some RFID reader configurations, interference can occur between the sending and receiving portions of the reader. This can be influenced by a number of different factors, but inevitably results in degradation of the reader's receiver sensitivity. Therefore, systems and methods which alleviate this problem would be particularly beneficial to the field of RFID readers and tags, as well as other RF systems employing backscatter communications.